Author(s): C. Tanner Chris; Tom Headley; Judy-Ann Ansen
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Keywords: Constructed wetland; Floating island; Heavy metals; Suspended sediment; Treatment pond; Stormwater; Urban pollution
Abstract: Floating treatment wetlands (FTW) provide an innovative approach for treatment of urban runoff. They employ emergent plants growing on a buoyant mat at the water surface, rather than rooted in the bottom sediments. They are therefore able to tolerate the wide fluctuations in water depths typical of stormwater ponds. Plant roots hang beneath the floating mat providing a large surface area for biofilm growth and entrapment of fine suspended particulates. Because the plants are not rooted in the sediment, they are forced to acquire their nutrition directly from the water column, which may enhance rates of nutrient and element uptake into biomass. A series of mesocosm experiments (12 x0.7 m 3 tanks using 0.36 m 2 floating mats) were conducted to examine the effectiveness of FTWs for removal of copper, zinc and fine particulates from stormwater. The results indicate that the presence of a planted floating mat can substantially improve the removal of fine suspended particles and copper. The removal of dissolved zinc was also improved by the inclusion of a floating mat, although the effect of plants was less clear. Cu and Zn mass removal rates in the order of 3.8–6.4 mg m -2 d -1 and25–88 mg m -2 d -1 respectively for planted FTWs, compare favourably to removal rates reported for conventional surface flow and subsurface flow constructed wetlands at similar loading rates. Although not directly measured in the present study, the removal of particulate-bound metals is also likely to be high given that the FTWs removed approximately one third of the very fine suspended particulate load within three days.
Year: 2010